1. Field of the Invention
The present invention relates to an image display medium repeatedly rewritable thereon, and to an image forming apparatus using particles.
2. Description of the Related Art
Traditionally, various display techniques have been proposed as an image display medium repeatedly rewritable thereon, such as a Twisting Ball Display (bichromal particle rotational display) medium, an electrophoresis medium, a magnetophoresis medium, a thermal rewritable medium and a liquid crystal medium having a memorizing property. Although such display techniques are superb in image memorizing properties, problems have arisen insofar that it has not been possible to secure a white display like paper on a display screen only a low degree of contrast has been achieved.
On the other hand, as a display technique for solving the aforementioned problems by use of a toner, a technique has been proposed in which images are displayed by means of an image display medium in which a conductive colored toner and white particles are sealed between a pair of substrates which are equipped with electrodes and disposed so as the electrodes face each other (See Japan Hardcopy '99 dissertations, pp. 249-252). According to this technique, an image display is performed by a mechanism described below. First, charges are injected into a conductive colored toner by way of an electron transport layer provided on the electrode side of a non-display substrate. The conductive colored toner into which charges have been injected is thereby transferred to the display substrate side disposed by an electric field generated between the electrodes and positioned opposite to the non-display substrate. In these circumstances, images are displayed by the contrast produced between the conductive colored toner fixed on the inner side of the display substrate (on the side facing the non-display substrate) and the white particles present on the same surface.
In this display technique, the image display medium is entirely solid and is superior insofar that in principle the white display and the black display can be interchanged 100%. However, when the aforementioned technique is used, a conductive colored toner, which does not make contact with the electron transport layer provided on the electrode surface side of the non-display substrate, and another conductive colored toner, which is isolated from another conductive colored toner, are both present. These conductive colored toners are present at random between the pair of substrates and they are not transferred by an electric field because there is no charge injection. This has resulted in a problem of a decrease in the degree of contrast.
On the other hand, an image display medium has been proposed (See Japanese Patent Application Laid-Open (JP-A) No. 2001-312225) in which a particle group composed of plural types of particles, which have different colors and different charging properties from each other, and which are sealed between a pair of substrates, can move between substrates by means of the application of an electric field. A high degree of whiteness and contrast can be obtained with this display.
In the initial stages of a repetitive display of images, the composition of particles used according to this technique is superior in terms of white density, black density, and contrast of density; however, after repeated displays over a long period of time, a reduction in image density can lead to a decrease in density contrast, or a deterioration in image uniformity can result in an unevenness of images.
As a results of assiduous studies by the present inventors, it has been discovered that such problems stem from instability in the amount of charging, the result of frictional electrification between particles.